doc: adds alternative variable syntax using @ delimiters to the documentation

[kevin0x90]

This pull request fixes a part of the documentation regarding filtering and the use of variables.

In the documentation was only the information that variables can be used via ${...} but there is also the option to use @...@ syntax to reference variables.

This pull request extends the documentation with information about this alternative syntax.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004